This project utilizes behavioral and physiological techniques in the same animals to determine the functional role of olivocochlear bundle (OCB) efferents in auditory perception. Subjects will be monkeys (Macaca fascicularis). These will tie trained to respond on perceptual tasks using operant conditioning techniques applied in a standard yes-no adaptive tracking procedure. Perceptual processes to be examined are (1) monaural thresholds in quiet and under conditions of broadband noise masking, (2) frequency selectivity, (3) dynamic range and growth of loudness, and (4) central masking. These tasks were selected for study because they allow hypotheses of OCB function previously described in the auditory research literature to be evaluated. Baseline data for normal performance on one or more perceptual tasks will be collected for each animal. The animal will then undergo either a unilateral vestibular neurectomy (to remove the entire OCB projection to one ear) or near-midline section at the floor of the fourth ventricle (to eliminate only the crossed OCB projections). Following a recovery period, animals will be re-tested on the same tasks as earlier to determine the perceptual consequences of OCB loss. After behavioral testing has been completed, each animal will become the subject of an acute electrophysiological experiment to record single auditory nerve fiber responses, using the same stimuli and data collection paradigms that were used behaviorally. At the conclusion of the physiological recordings, the operated and unoperated cochleas of each animal will be injected with horseradish peroxidase and nuclear yellow, respectively, and light and electron microscopic techniques will be used to verify OCB transection (for the operated ear) and determine the number and condition of surviving OCB cells (for the unoperated ear).